Machine-readable optical disc with reading-inhibit agent

ABSTRACT

An optical disc having machine-readable, information-encoding features is provided with a barrier layer secured to the disc. This barrier layer is configured to prevent machine-reading of the features. A reading-inhibit agent, included in the disc and activated by removal of the barrier layer, is operative, once activated, to alter the disc to inhibit reading of the disc. Alternately, the barrier layer can be eliminated, and the reading-inhibit agent can be activated by initial reading of the disc, as for example by exposure to optical radiation associated with reading of the disc, or rotation of the disc.

This application claims the benefit of U.S. Provisional Application No.60/026,390, filed Sep. 16, 1996.

BACKGROUND OF THE INVENTION

This invention relates to machine-readable optical discs of all types,including for example digital discs such as compact discs (CD's),digital video discs (DVD's), CDROM's, and the like.

Conventional optical discs have reached widespread acceptance as alow-cost, reliable storage medium for digital information includingmusic, video, and data. One of the traditional advantages of opticaldiscs is their long life.

However, in some applications, the long life of the conventional opticaldisc may represent a disadvantage. For example, if music, moviessoftware is to be rented for a limited time period, the original opticaldisc must be returned at the end of the rental period.

A need presently exists for an improved machine-readable optical discthat eliminates the need for the return of an optical disc at the end ofa rental period.

SUMMARY OF THE INVENTION

According to a first aspect of this invention, an optical disccomprising machine-readable, information-encoding features is providedwith a barrier layer releasably coupled to the disc. This barrier layeris configured to prevent machine-reading of the disc. A reading-inhibitagent is included in the disc, and is activated by removal of thebarrier layer. This reading-inhibit agent is operative, after it isactivated, to alter the disc to inhibit reading of the disc. Both thebarrier layer and the reading-inhibit agent can take many forms, asdiscussed by way of example below.

According to another aspect of this invention, an optical disccomprising machine-readable, information-encoding features is providedwith a reading-inhibit agent that is activated by machine-reading thedisc. This reading-inhibit agent is operative, after it is activated, toalter the disc to inhibit reading of the disc. In alternate embodiments,the reading-inhibit agent may be activated by optical radiation incidenton the disc during machine-reading of the disc, or by rotation of thedisc during machine-reading of the disc.

According to a third aspect of this invention, a method is provided forinhibiting reading of an optical disc. According to this method, anoptical disc is provided comprising machine-readable,information-encoding features, and a reading-inhibit agent. Thereading-inhibit agent is activated by optical radiation, and isoperative, once activated, to alter the disc to inhibit reading. Areading device is provided to read the disc, and this reading devicecomprises a source of optical radiation. According to the method of thisinvention, the disc is read with the reading device, and the inhibitagent is concurrently activated with optical radiation from the source.The source of optical radiation that activates the reading-inhibit agentcan either be the source of optical radiation that forms the readingbeam, or a second source, separate from the reading beam source.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 3 are partial cross-sectional views of three barrierlayers suitable for use in embodiments of this invention.

FIGS. 4, 5, 6, 7 and 8 are partial cross-sectional views of opticaldiscs that incorporate first, second, third, fourth, and fifth preferredembodiments of this invention, respectively

FIGS. 9 and 10 are plan views of optical discs that incorporate sixthand seventh preferred embodiments of this invention, respectively.

FIGS. 11 and 12 are partial cross-sectional views of optical discs thatincorporate eighth and ninth preferred embodiments of this invention,respectively.

FIG. 13 is a plan view of an optical disc that incorporates a tenthpreferred embodiment of this invention.

FIGS. 14 and 15 are partial cross-sectional views of optical discs thatincorporate embodiments of the invention employing galvanic cells.

FIG. 16 is a partial cross-sectional view of a prior art compact disc.

FIG. 17 is a partial cross-sectional view of a disc containing areservoir.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The present invention can be implemented in many different ways, and thefollowing discussion will describe selected embodiments of theinvention. These embodiments are intended as examples only, and not asan exhaustive list of all forms that the invention can take. Generallyspeaking, the embodiments discussed below can be classified into twogroups. The first group uses a barrier layer to prevent prematureactivation of the reading-inhibit agent, while the second group does notuse such a barrier layer.

In general, this invention can be used with the widest possible varietyof optical discs comprising machine-readable, information-encodingfeatures. FIG. 16 shows a highly schematic cross section of an opticaldisc such as a prior art compact disc. FIG. 16, like all of the otherfigures, is not drawn to scale; selected features have been exaggeratedin size for clarity of illustration. The disc of FIG. 16 includes asubstrate 10 which is formed with an array of information-encodingfeatures such as pits 12. The surface defining the information-encodingfeatures 12 is covered with a reflective layer 14, which may be, forexample, formed of aluminum. The reflective layer 14 is in turn coveredwith a protective layer 16 which protects the reflective layer 14 fromoxidation and physical damage. A reading beam aligned with the arrow 18is incident on the surface of the substrate 10 opposite theinformation-encoding features 12. This reading beam passes through thesubstrate 10, is reflected by the reflective layer 14, and then passesout through the substrate 10 for detection. Features 10-18 describedabove are completely conventional. As used herein, the term"information-encoding features" is intended broadly to encompass thewidest possible range of such features, regardless of the particularencoding mechanism or reading beam interaction mechanism that is used.

Embodiments that Utilize a Barrier Layer

The following embodiments of the invention utilize a barrier layer toprevent activation of the reading-inhibit agent until the barrier layerhas been removed. FIGS. 1-3 show three different types of barrier layersthat can be used. In FIGS. 1-3, the reference symbol 20 is used todepict the optical disc, which includes information-encoding features 22on the upper surface of the disc, in the orientation shown in thefigures. In the embodiment of FIG. 1, a barrier layer 24 is releasablysecured (as for example with a suitable adhesive) adjacent the surfaceof the optical disc 20 that carries the information-encoding features22. In the embodiment of FIG. 2, the barrier layer 26 is releasablysecured to the surface of the disc 20 opposite the surface that carriesthe information-encoding features 22. In the embodiment of FIG. 3, thebarrier layer 28 is formed as a closed package which completely sealsthe optical disc 20 from contact with ambient oxygen and moisture. Inthis case, there is no need for the barrier layer 28 to be adhesivelysecured to the disc 20. As used herein, a barrier layer which isreleasably coupled to an optical disc may be coupled adhesively as shownin FIGS. 1 and 2, coupled by enveloping the disc as shown in FIG. 3, orcoupled in any other way that reliably associates the barrier layer andthe disc prior to removal of the barrier layer.

As pointed out below, the reading-inhibit agent can take many forms andcan be applied at many different places on the optical disc 20.

Depending upon the reading-inhibit agent used and its location, theposition and physical and chemical characteristics of the barrier layer24, 26, 28 can be selected as appropriate.

It is not essential in all applications that the barrier layer cover anentire surface of the disc 20. If the reading-inhibit agent is localizedto a particular portion of the disc, the barrier layer may cover only anarea adjacent to and aligned with that portion. Preferably, the barrierlayer should prevent machine-reading of the optical disc until it isremoved.

Reading-inhibit Agents that Disrupt Readability of the Optical Disc byControlled Degradation of the Reflective Layer

A first type of reading-inhibit agent disrupts the reflectivity of thereflective layer in optically read discs to such an extent that theencoded data is rendered unusable. By disrupting the readability of thedisc at a known time after the initial use of the disc, or after removalof the barrier layer, the practical usage lifetime of the disc can belimited and controlled.

The reflective layer 14 that is conventionally used in optical discs istypically formed as a thin film of metallic aluminum. This aluminum filmcan be corroded by exposure to an oxidizing environment to such anextent that the film no longer has sufficient reflectivity to supportoptical reading of the disc. For example, water and oxygen from theatmosphere can form a suitable oxidizing environment for such analuminum film. The rate and timing of the corrosion of the aluminum filmcan be controlled by several approaches, including control of theconcentration of an oxidizing species, control of the solution pH,introduction of dissimilar metal couples, and introduction of chemicalspecies to control solubility of aluminum. For example, in the casewhere atmospheric oxygen is the oxidant, a porous polymer film may beplaced over the aluminum film to provide known permeabilitycharacteristics for moisture and oxygen from the atmosphere as itmigrates to the aluminum film. In this case, corrosion can besubstantially prevented by a barrier layer such as the barrier layer 24of FIG. 1 or the barrier layer 28 of FIG. 3 until the barrier layer isremoved prior to initial reading of the optical disc.

A key feature of optically read discs is the use of a reflective layer14 as described above to reflect light from the interrogating lightsource, generally a laser operating with a principal wavelength in thevisible portion of the spectrum, to the detector. The reflective layer14 is most generally composed of metallic aluminum which is deposited onto the information-encoding features by sputtering a very thin film.This thin film is approximately 55 nanometers in thickness inconventional compact discs.

Conventional reflective layers are subject to corrosion reactionsinvolving oxidation of the metallic aluminum and subsequent formation ofaluminum compounds such as hydroxy salts which are not reflective:

    Al→Al.sup.+3 +3e.sup.-  (oxidation),

    Al.sup.+3 3OH.sup.- →Al.sup.+3 (OH).sub.3 (compound formation).

The oxidation of the aluminum metal is balanced by a reduction reactionsuch as the following.

    O.sub.2 +2H.sub.2 O+2e.sup.- →4OH.sup.-  (in neutral or alkaline solutions),

    2H.sup.+ 2e.sup.- →H.sub.2 (in acidic solutions).

The corrosion reaction typically involves an electrolyte film on thesurface of the aluminum to form an ionic path between the oxidation andreduction sites on the aluminum surface. In the example of atmosphericoxygen, a film or layer of water on the surface is one suitableelectrolyte. The rate of corrosion will be influenced by theavailability of the oxidizing species (e.g. oxygen or hydronium, H+),the addition of soluble salts to influence the conductivity of theelectrolyte, the addition of chlorides to alter the stability of thenormally protective aluminum oxide film, pH buffers to influence thestability of the normally protective aluminum oxide layer or toinfluence the reduction reaction, or the addition of complexing agentsto dissolve protective aluminum oxides or to keep aluminum corrosionproducts in solution. Such salts and other complexing agents may bedeliberately added in a layer of material placed next to the aluminumlayer. Addition of a hygroscopic material and salts to this layer canalso aid in collecting atmospheric moisture for subsequent release asliquid water solution at the corrosion reaction site. The hygroscopicmaterial or salts effectively lower the dew point of the aluminumsurface, the relative humidity at which a liquid film forms on the metalsurface.

Cupric and ferric chloride are specific examples of oxidizers that maybe incorporated into an electrolyte layer next to the aluminum layer toaccelerate corrosion of the aluminum. These materials offer severaladvantages. If the metal cation reduced to the metallic state in theoxidation reaction, the metal (e.g. copper or iron) deposited on thealuminum surface forms local cathodes that can accelerate corrosion ofaluminum in adjacent areas. If the metal cation is not completelyreduced to the metallic state, the cuprous or ferrous species may reactwith oxygen to restore the oxidizing power of the solution.

FIG. 4 shows one preferred embodiment of this invention which includes asubstrate 10 and a reflective layer 14 as described above. In this case,an electrolyte layer 30 is applied adjacent to the reflective layer 14.The electrolyte layer 30 contains substances which aid the corrosionreactions, such as hygroscopic salts, pH buffers, complexing agents foraluminum, and the like. The electrolyte layer 30 is in turn covered withan outer layer 32 of a material which is permeable to environmentalmoisture and oxygen. The permeable layer 32 is in turn initially coveredby a barrier layer 24 as described above. The barrier layer 24 preventsoxygen and water from reaching the permeable layer 32 during storage andtransport. When a user wishes to read information from the optical discof FIG. 4, the user removes the barrier layer 24. Oxygen and water vaporfrom the atmosphere then diffuse through the permeable layer 32 at acontrolled rate. The water vapor can be, for example, collected byhygroscopic materials in the electrolyte layer 30, and subsequently madeavailable to aid in the aluminum corrosion reactions discussed above.

Based on typical corrosion rates for aluminum, and an assumed reflectivelayer thickness of 55 nanometers, the reflective layer may be degradedadequately to prevent machine-reading of the optical disc in, forexample, 1 to 100 hours after removal of the barrier layer 24, dependingupon the availability of moisture, and the parameters of the electrolytelayer 30 and the permeable layer 32.

Table 1 illustrates the relationship between the corrosion ratei_(corr), the rate of aluminum film removal L, and the time t.sub.(55nm) to corrode 55 nanometers of aluminum. In Table 1, L is estimatedusing Farady's law.

                  TABLE 1                                                         ______________________________________                                        i.sub.corr      L       t.sub.(55 nm)                                         (uA/cm2)        (nm/hr) (Hours)                                               ______________________________________                                        0.1             0.1     442.3                                                 1                                         44.2                                10                                       4.4                                  1.00                                    0.4                                   ______________________________________                                    

If desired, metallic films or pieces of a more noble metal (for examplea metal such as copper or silver, or carbon) can be placed in electricalcontact with an aluminum reflecting layer 14 and with an electrolytelayer 30 containing oxygen as described above or other suitableoxidizing species. In this case the galvanic couple due to the presenceof the more noble element will result in more rapid and directedcorrosion of the aluminum reflecting layer 14 than would otherwise occurin the absence of that second, more noble element.

Additionally, if desired the reflective layer 14 can be sputter-coatedin such a manner that the reflective layer 14 itself includes more nobleelements such as copper in the reflective film itself. The aluminumalloy film will have a higher corrosion rate than a purer aluminum filmdue to the formation of localized cathodes at the sites of the morenoble elements.

FIG. 14 is a schematic view of an optical disc 80 which includes analuminum layer 82 and a copper layer 84, separated by an electrolytelayer 86. The metal layers 82, 84 may be configured for example as aconventional two-sided DVD to encode information, and the copper layer84 provides sufficient reflectivity for conventional reading. The metallayers 82, 84 are connected electrically in any convenient manner, forexample by a metal foil 88 or a conductive adhesive (e.g. an epoxyfilled with carbon, silver or copper particles). The three layers 82,84, 86 and the foil 88 form a galvanic cell, in which the aluminum layer82 is the anode that corrodes relative to the more noble metal. Theelectrolyte layer 86 provides ionic continuity between the layers 82,84, while the foil 88 provides electronic contact.

FIG. 15 shows an optical disc 80' that is similar to the disc 80 of FIG.14. Primed reference numerals are used in FIG. 15 for elementscorresponding to elements 82-88 of FIG. 14. In FIG. 15 the area of thecopper layer 84' is greater than the area of the aluminum layer 82' toincrease the aluminum corrosion rate. Also, openings 90' are providedthrough the copper layer 84' and the adjacent polycarbonate layer 92' tofurther increase the aluminum corrosion rate. Preferably, the openings90' are located in an area of the disc 80' not containing storedinformation, such as the central portion of the disc 80'.

As shown in FIG. 5, it is not essential in all embodiments thatatmospheric oxygen and water be used as the oxidizing species. Forexample, as shown in FIG. 5, microcapsules 34 can be provided betweenthe barrier layer 24 and the permeable layer 36. These microcapsules cancontain any suitable oxidizing species and electrolyte. In this exampleremoval of the barrier layer 24 ruptures at least some of themicrocapsules 34, thereby releasing electrolyte and oxidant into thepermeable layer 36 The electrolyte and oxidant migrate through thepermeable layer 36 and come into contact with the reflective layer 14 inorder to initiate a controlled corrosion process. This embodiment isless sensitive to the availability of atmospheric moisture than theembodiment of FIG. 4.

From the foregoing it should be apparent that the reading-inhibit agentcan take many forms, including electrolytes, oxidizing species, variouselements more noble than the reflective metal, and permeable films thatcontrol the rate at which atmospheric oxygen and water reach thereflective layer. In various embodiments the inhibit agent can take theform of films, or it can be contained in various ways, including by useof microcapsules.

The following paragraphs detail test results related to the use ofhygroscopic salts, placed on an aluminum surface, to pick up water fromthe atmosphere and form an electrolyte film. The hygroscopic salts maybe sufficiently corrosive by themselves, or alternately they may be usedin conjunction with other salts and complexing agents to provide thedesired aluminum removal rate. The salts are preferably applied in theanhydrous form to the surface, and are then protected by a barrier toexclude moisture from the salts. Activation of the corrosion processoccurs when the barrier is removed.

The corrosion approach is based on the principle that a dry salt willcome to equilibrium with its environment. In the process of coming toequilibrium, the salt can either dissolve, to form an electrolytesolution, or become drier. Table 1a lists the humidity above saturatedsolutions of several salts in a closed environment. If the salt isplaced in air with higher humidity than the table value, it will pick upwater. If the humidity is lower than the table value, the solution willlose water. The salts used in this application include magnesiumchloride and quaternary ammonium amine chlorides.

                  TABLE 1a                                                        ______________________________________                                        Humidity Above Saturated Solutions of Various Salts                           Solid Phase     t ° C.                                                                        % Humidity                                             ______________________________________                                        H.sub.3 PO.sub.4.1/2H.sub.2 O                                                                 24     9                                                      LiCl.H.sub.2 O                        15    20                                KC.sub.2 H.sub.3 O.sub.2                                                                                            20                                      Pb(NO.sub.3).sub.2                                                                                                  9820                                    ______________________________________                                    

Lithium chloride and potassium acetate were tested as the candidatesalts. To these, either potassium hydroide (KOH) or trisodium phosphate(TSP) were added to increase the aggressiveness of the electrolyte.Placement of dilute solutions of either KOH or TSP on the disc surfacequickly dissolved the aluminum film. With these aggressive salts,complexing agents, such as citrate, were not needed to remove anypassive films on the aluminum.

Further, tests were conducted by placing the salts onto the unprotectedaluminum layer of CDs. Some of the CDs were then left exposed to roomair while others were placed in desiccators with relative humidities of20% and 8.5%. The relative humidities in the desiccators were controlledby solutions of sulfuric acid; the specific gravity of the sulfuric acidsolution was selected to provide the desired relative humidity. Duringthese experiments, ambient relative humidities ranged from 20 to 30percent. Four salts were used: potassium acetate (KAc), lithium chloride(LiCl), KOH, and TSP and were mixed as shown in Table 1b. Theconcentration of salt in the solution on the disc surface depended onthe amount of water that was absorbed.

                  TABLE 1b                                                        ______________________________________                                        Salts Mixtures                                                                SALT         TSP         KOH                                                  ______________________________________                                        KAc (4 grams)                                                                              1.31 g or .13 g                                                                           0.58 g or 0.06 g                                     LiCl (4 grams)                                                                                   1.31 g or 0.13 g                                                                         0.58 g or 0.06 g                                ______________________________________                                    

When LiCl was placed on the disc's aluminum surface under ambientconditions, droplets of water formed on the salt mass within 30 minutes;with KAc it took 3 hours. The water droplets formed with LiCl wereclearly visible to the unaided eye; the droplets formed with KAc couldbe observed with the use of a magnifying glass. After these samples wereallowed to stand overnight, the aluminum with LiCl showed partialcorrosion, while the aluminum with KAc was intact.

The tests also showed that KOH alone was highly hygroscopic and corrodedthe discs under all conditions. Within the limitations of existingequipment, under the driest conditions KOH corroded the aluminum surfacein all tests. The water retained in the KOH was sufficient to corrodethe aluminum surface, even when a glove bag was used to apply the KOH,and a dry desiccator was used to store the sample.

At 20% RH, the LiCl (alone and in mixtures) continued to form waterdroplets on the disc surface and to attack the aluminum. In the 8.5% RHdesiccator, visible water droplets did not form, in agreement with thetable values.

TSP did not attack the aluminum when placed on the surface by itself,even under ambient conditions. TSP was not sufficiently hygroscopic toform an aggressive electrolyte film. However, when used in conjunctionwith LiCl at 20% RH, enough water was picked up to form an aggressivesolution, which attacked the aluminum. A mixture of LiCl and TSP did notattack the aluminum in the 8.5% RH desiccator (no breakthrough afterfour days).

These tests demonstrated that the corrosion process can be activated byambient moisture down to at least 20% relative humidity, and probablydown to 15% based on published values for LiCl. Other salts or drier KOHmay allow one to go to even lower humidities.

Reading-inhibit Agents that Operate by Absorbing Optical Radiation ofthe Reading Beam

The digital video disk (DVD) format uses a 650 nm laser to readinformation from the disk. If this reading beam is absorbed to asignificant degree, the return signal from the disk is attenuated. Byincluding a light-absorbing material in the disk, it is possible toattenuate the reading signal enough to prevent the disk from being read.Preferably, the light-absorbing material is strongly absorbing at thewavelength of the reading beam. Many compounds absorb at 650 nm, andthey usually appear blue or green in color.

In order to allow the disc to be read on its first use, thelight-absorbing material is initially nonabsorbent at the wavelength ofthe reading beam. Over time, for example four to 24 hours, thislight-absorbing material becomes absorbing at the wavelength of thereading beam in response to some environmental stimulus. One approach isto use a compound for the light-absorbing material that is initiallycolorless, but which oxidizes to a new compound which is colored uponexposure to oxygen in the atmosphere, or some other oxidant. Manycompounds are known which exhibit this behavior. Four compounds whichmay be particularly appropriate are given in Table 2 (in their oxidizedform).

                  TABLE 2                                                         ______________________________________                                        Compound       Color Index Number                                             ______________________________________                                        Indigo Carmine 73015                                                          Methylene Blue                        52015                                   Thionin                                      52000                            Gallocyanine                            51030                                 ______________________________________                                    

The colorless precursor to the light-absorbing material is incorporatedin the optical disc somewhere along the path taken by the laser light ofthe reading beam. For instance, the colorless precursor can becompounded within the material (typically polycarbonate) that makes upthe substrate 10, or the colorless precursor can be included in acoating on a surface of the substrate 10.

Preferably, the rate at which atmospheric oxygen reaches the colorlessprecursor is controlled in order to render the optical disc unreadableat a selected time after the barrier layer is removed. The rate at whichoxygen reaches the colorless precursor should be selected such that theoptical disc can be read at least once before sufficient color isgenerated to make the optical disc unreadable. The rate at which oxygenreaches the colorless precursor should be high enough to ensure that theoptical disc becomes unreadable within the desired time period (forexample four to 24 hours). Various methods can be used to control therate at which oxygen reaches the colorless precursor. If thelight-absorbing compound is contained within the body of the substrate107 the amount of the absorbing compound can be adjusted as appropriatefor the application; higher loadings will result in quicker obscuration.The rate at which the absorbing compound becomes absorbing to thereading beam can be lowered by lowering the concentration of theabsorbing compound in the substrate, or by applying an outer coating tothe substrate which acts as a semipermeable oxygen barrier.

Alternately, the absorbing compound can be placed as shown in FIG. 6 ina layer 38 on a surface of the substrate 10. The rate of the oxidationreaction can be controlled in this case by choosing a matrix such as asuitable polymer for the absorbing compound layer having the appropriatebarrier properties. Alternately, an additional coating layer can beemployed over the absorbing layer, and this additional coating can actas a semipermeable oxygen barrier which allows oxygen to reach theabsorbing layer at the desired rate.

As shown in FIG. 6, a barrier layer 26 is used to protect the absorbinglayer 38 from atmospheric oxygen during storage and transport. Thebarrier layer can also take the form of an air-tight package, as shownin FIG. 3.

Reading-inhibit Agents that Operate by Altering Physical Dimensions ofthe Optical Disc

Certain embodiments of the invention use a reading-inhibit agent whichalters its physical dimension when activated. A superabsorbing polymeris one such material, for example a polymer or copolymer containing acarboxylic or alcohol moiety. For example, a water-absorbent resin maybe formed from a cross-linked polymer or a copolymer of acrylic acid,methacrylic acid, methylacrylate-vinylacetate, starch-ethyl acrylate,starch-acrylonitrile, carboxymethyl cellulose, ethylene oxide, vinylalcohol, acrylamide, and the like.

Such materials can be used in several ways to make an optical discunreadable, for example as the material absorbs ambient moisture. Theabsorption of such moisture creates a volume change in the material,which can be used to cause a combination of any of the following effectsto prevent reading: delamination, a change in the refractive index, or achange in spinning characteristics.

For example, as shown in FIG. 7, a superabsorber layer 42 can be placedbetween two digital video disc substrates 40. The entire digital videodisc is then protected with an encapsulating barrier layer 28 similar tothat shown above in FIG. 3. When the barrier layer 28 is removed,ambient moisture is allowed gradually to reach the superabsorber layer42. As the superabsorber layer absorbs moisture, it will increase involume, thereby causing the digital video disc to delaminate andpreventing further reading of the disc.

In the example of FIG. 8, a superabsorber layer 44 is placed on thereadable surface of a digital video disk 40, and this superabsorberlayer is protected by a barrier layer 26. When the barrier layer 26 isremoved, the superabsorber layer 44 will absorb ambient moisture andincrease in volume. This volume increase causes a significant change inthe refractive index of the material, which renders the digital videodisc unreadable.

As shown in FIG. 9, a superabsorber layer 48 may be placed eitherpartially or completely around a spindle hole 46 of the digital videodisk 40. This superabsorber layer 48 is protected by a barrier layer(not shown in FIG. 9) prior to use. When the barrier layer is removed,ambient moisture will gradually cause the superabsorber layer 48 toexpand. If the superabsorber layer 48 is placed as shown in FIG. 9, thiscan cause the spindle hole 46 to assume an eccentric position, therebyrendering the optical disc unreadable. Alternately, if the superabsorberlayer 48 extends substantially around the spindle hole 46, thesuperabsorber layer 48 may expand to the point where the spindle hole 46is too small to fit on the spindle of the reading device.

FIG. 10 shows another embodiment in which the superabsorber layer 50 ismounted near the outer rim of the digital video disk 40. As before, thesuperabsorber layer 50 is initially protected by a barrier layer (notshown in FIG. 10). Once the barrier layer is removed, the superabsorberlayer 50 absorbs atmospheric moisture, thereby rendering the discsufficiently out of balance to prevent reliable reading.

In all of the examples discussed above, the rate at which thesuper-absorber layer absorbs moisture can be modified by placing asemipermeable barrier over the exposed surface of the superabsorberlayer. This barrier can regulate the diffusion of ambient moisture tothe superabsorber layer, which in this way controls the time periodduring which the optical disc is readable after the barrier layer hasbeen removed.

Reading-inhibit Agents that Operate by Scattering the Reading Beam

As discussed above, a laser beam is typically used as a reading beam foroptical discs. If the reading beam is scattered or otherwise attenuatedto a significant degree, the disc cannot be accurately read. Forexample, as shown in FIG. 11, a digital video disc 40 can be providedwith a layer 52 that includes a material such as a solvent that willalter the optical characteristics of the adjacent portion of the digitalvideo disc 40. For example, a polycarbonate exposed to solvent is knownto craze, i.e. to form a diffuse, opaque film or layer, which scattersthe reading beam. Suitable solvents include organic liquids or vaporssuch as acetone, xylene and the like. Depending upon the concentrationof the solvent and the exposure time, various rates of loss oftransparency can be obtained. Other coatings in addition topolycarbonates can exhibit the same effective behavior by slightdissolution in an organic solvent followed by deposition on the surfaceof the disc as the solvent evaporates or is lost. The redepositionprocess may also include a recrystalization of a glassy coating layer.This redeposition results in a less transparent and therefore lessreadable surface on the disc. The layer 52 of FIG. 10 can includemicroencapsulated solvent beads which will rupture on removal of theadjacent barrier layer 26.

Embodiments that Include Reading-inhibit Agents Without Barrier Layers

As pointed out above, it is not essential in all embodiments that abarrier layer be included. Rather, in some embodiments it is the act ofreading the disc that activates the reading-inhibit agent. For example,optical radiation associated with disc reading, or rotation associatedwith disc reading can activate the reading-inhibit agent.

As shown in FIG. 12, one such embodiment includes an optical disc 54which includes a reading-inhibit agent 56 adjacent one surface. In thiscase the reading-inhibit agent 56 is a photoactive material that, whenactivated by suitable optical radiation, is suitably changed in opticalor physical characteristics so as to inhibit further reading of thedisc. The photoactive material can alternately be dispersed in the bulkof the disk and can for example change from clear to opaque at thewavelength of the reading beam upon exposure to suitable opticalradiation. As shown in FIG. 12, the disc 54 is installed in a readingdevice 58. The reading device 58 includes a first optical source such asa laser 60 that directs the reading beam 62 against the disc 54.Returning radiation from the disc 54 is sensed by a detector 64, in theconventional manner. In this embodiment, the reading device 58 furtherincludes a second optical source 66. The second optical source 66destroys or degrades the optical transmission or reflection required toread the disc. The second source 66 may be a conventional source such asa high pressure arc, an incandescent bulb, a fluorescent lamp, or alaser. As the disc 54 is read, radiation from the second source 62interacts with the reading-inhibit agent 56 to inhibit further readingof that portion of the disc 54. The second source 62 is arranged suchthat the second source 62 does not illuminate any portion of the disc 54until after that portion of the disc 54 has been read by the readingbeam 62.

In alternate embodiments the reading beam 62 itself may initiate opticalchanges in the read inhibiting agent 56, thereby dispensing with theneed for the second source 62.

Alternately, when the second source 62 is used, the need for a separateread inhibit agent 56 may be eliminated. In this case, the second source66 may for example be a passively q-switched microchip laser focused onthe surface of the disc. The effect of this laser is to createscattering centers by ablating the read surface of the disc. Thescattering centers reduce the optical transmission of the disc to thereading beam 62.

In either case, the second source 66 should be interlocked in a way thatprevents consumer tampering, and should track in a way so as not tointerfere with the initial reading of the disc. When the second source62 is of sufficient power to provide the ablating action describedabove, access to the information on the disc will be denied almostimmediately after it is read.

FIG. 13 shows another embodiment having a reading-inhibit agent which isactivated by the act of reading the disk. In this case an optical disc70 includes a reservoir 72 that contains a reading-inhibit agent, suchas a suitable solvent. The reservoir 72 includes an opening 74. When thedisc is first rotated in order to be read, solvent passes out of thereservoir 72 via the opening 74, and in this way a small quantity ofsolvent is released to the disc. The solvent can degrade the opticalcharacteristics of the disc, as discussed above, to prevent reading ofthe disc a predetermined time after the solvent has left the reservoir.As one example, the reservoir 72 may be formed in a region bounded bytwo concentric annular ridges, similar to the stacking ringsconventionally used in current optical discs.

Additional Embodiment

FIG. 17 shows a cross-sectional view that illustrates one form of a disc100 containing a reservoir 102 as discussed immediately above. One ormore capillary-tube-sized passages 104 are radially oriented to allow asuitable reading-inhibit agent (such as a solvent or a corrosive agentas discussed above) to flow from the reservoir 102 radially outwardly tothe region of the disc that stores information via information-encodingfeatures. The reservoir 102 and the passage 104 are closed by a siliconemembrane 108 that defines an array of vents 110, 112. In this example,the vents 110, 112 are formed as pin pricks. The silicone membrane 108is covered by a polycarbonate sheet 114 that defines vents on 116, 118aligned with the vents 110, 112, respectively.

A releasable, peel-off label 120 is removably secured by a suitableadhesive to the polycarbonate layer 114. This peel-off label 122includes a tab 122 to facilitate removal and a protrusion 124. Theprotrusion 124 passes through an opening in the polycarbonate layer 114and presses the silicone membrane 108 into the passage 104 to create amechanical valve that stops the flow of reading-inhibit agent radiallyoutwardly from the reservoir 102. Optionally, the passage 104 may alsoinclude a valve element 106 of a material that is dissolved by thereading-inhibit agent. For example, a valve element 106 of aluminum canbe used in cases where the reading-inhibit agent is corrosive toaluminum. Preferably, the reservoir 102 includes a wick 103 made ofcotton or microfiber to retain fluid in the reservoir 102. The passage104 may have a cross-sectional size of 0.02 inch.

Preferably, the peel-off label 120 is sized such that the label must beremoved in order to allow the disc 100 to be read. Once the label 120has been removed, the vents 110, 112 are opened, and the protrusion 124is removed. This allows the silicone membrane 108 to relax upwardly,thereby opening the passage 104. When the disc 100 is rotated during areading operation centrifugal force causes the reading-inhibit agent inthe reservoir 102 to flow radially outwardly via the passage 104 ontothe information-encoding portion of the disc 100.

In some embodiments the reading-inhibit agent may be selected so as notto interfere with normal reading of the disc 100 until a selected timeafter the reading-inhibit agent has contacted the information carryingportion of the disc. As an alternative, when the optional valve element106 is used, the valve element 106 prevents the reading-inhibit agentfrom reaching the information carrying portion of the disc 100 until thevalve element 106 is dissolved by the reading-inhibit agent. In thisway, the plug 106 provides a timed release of the reading-inhibit agentonto the information carrying portion of the disc.

Tests have shown that two-pass transmission of the disc typically mustfall to about 45 percent of the original value before a significantnumber of reading errors occur, and to approximately 30 percent of theoriginal value before the disc becomes unplayable.

Conclusion

The optical discs described above have a short effective life, limitedeither by the number of times the disc is played (e.g. one, two or moretimes), or by the passage of time after the disc is dispensed (e.g. aselected number of hours after the disc is sold or rented, after theconsumer opens a package, or after the disc is inserted into a discplayer). The effective life of the disc may be limited in response toreading of the disc, opening of the disc, or rotation of the disc.Various methods for limiting the effective life of the disc have beendescribed, including physical, chemical, and electrochemical methods.Physical methods include the diffusion of air or a component of air suchas oxygen, resulting in physical and/or chemical effects; the use ofoptical activation to cause a physical change in the disc; or the use ofphysical forces or the removal of forces associated with rotation of thedisc or removal of a label to cause a physical change in the disc.Chemical methods include a layer of the disc interacting with a chemicalapplied when the package is opened or by the vendor at the time of sale.Electrical or electrochemical methods include the use of anelectrochemically active system to accelerate corrosion.

It should be apparent from the foregoing detailed description that thepresent invention can be implemented in a wide variety of forms. Barrierlayers can take the form of sheets or patches on a surface of the disc,or of encapsulating packaging. In some cases barrier layers are notrequired. Reading-inhibit agents can take many forms, includingmaterials which change optical or physical characteristics of thereflecting layer, or various other components of the optical disc.Reading-inhibit agents can be employed as microencapsulated materials,materials formed in layers over selected regions of a disc, or materialsincorporated into other components of a disc. Reading-inhibit agents mayextend over the entire information-encoding surface of the optical disc,or alternately may be limited to selected portions, for example portionsthat encode indexing or other introductory information.

It should therefore clearly be understood that the foregoing detaileddescription is intended by way of illustration, not limitation. It isonly the following claims, including all equivalents, that are intendedto define the scope of this invention.

We claim:
 1. An optical disc comprising:a substrate; at least one layersupported by the substrate and having machine-readable,information-encoding features; a reading-inhibit agent, included in thedisc and activated by machine-readable the disc, saidinformation-encoding features being machine-readable prior to activationof the reading-inhibit agent, said reading-inhibit agent operative, onceactivated, to alter the disc to inhibit further reading of the disc. 2.The invention of claim 1 wherein the inhibit agent is activated byoptical radiation incident on the disc during machine-reading of thedisc.
 3. The invention of claim 1 wherein the inhibit agent is activatedby rotation of the disc during machine-reading.
 4. The invention ofclaim 1 further comprising a reservoir on the disc containing thereading-inhibit agent, said reservoir configured to release thereading-inhibit agent when the disc is rotated during machine-reading.5. An optical disc comprising:a substrate; at least one layer supportedby the substrate and having machine-readable, information-encodingfeatures; a reservoir included in the disc; a passageway interconnectingthe reservoir and a portion of the disc comprising theinformation-encoding features; and a reading-inhibit agent, included inthe reservoir and automatically activated by machine-reading the disc,said information-encoding features being machine-readable prior toactivation of the reading-inhibit agent, said reading-inhibit agentoperative, once activated, to alter the disc to inhibit further readingof the disc.
 6. The invention of claim 5 wherein the passagewaycomprises a valve.
 7. The invention of claim 6 wherein the valvecomprises a valve element that is soluble in the reading-inhibit agent.8. The invention of claim 6 wherein the valve comprises a mechanicalvalve.
 9. The invention of claim 7 comprising a wick disposed in thereservoir.
 10. The invention of claim 7 further comprising at least onevent in communication with the passageway.
 11. The invention of claim 1or 5 wherein the reading-inhibit agent, once activated, provides a shorteffective life for the disc.
 12. The invention of claim 1 or 5 whereinthe reading-inhibit agent, once activated, provides a short practicalusage lifetime for the disc.
 13. A method for inhibiting reading of anoptical disc, comprising the following steps:(a) providing an opticaldisc comprising a substrate, at least one layer supported by thesubstrate and having machine-readable, information-encoding features,and reading-inhibit agent, said information-encoding features beingmachine-readable prior to activation of the reading-inhibit agent, saidinhibit agent activated by optical radiation and operative, oneactivated, to alter the disc to inhibit further reading; (b) providing areading device operative to read the disc, said reading devicecomprising a source of optical radiation; and (c) reading the disc withthe reading device and concurrently activating the inhibit agent withoptical radiation from the source.
 14. The method of claim 13 whereinthe reading device provided in step (b) additionally comprises a sourceof a reading beam, in addition to the source of optical radiation.
 15. Amethod for inhibiting reading of an optical disc, comprising thefollowing steps:(a) providing an optical disc comprising a substrate, atleast one layer supported by the substrate and having machine-readable,information-encoding features, and a reading-inhibit agent, saidinformation-encoding features being machine-readable prior to activationof the reading-inhibit agent, said inhibit agent activated by opticalradiation and operative, one activated, to alter the disc to inhibitfurther reading; (b) providing a reading device operative to read thedisc, said reading device comprising first and second sources of opticalradiation; and (c) reading the disc with the first source andconcurrently activation the inhibit agent with the second source. 16.The method of claim 13 or 15 wherein the reading-inhibit agent, onceactivated, provides a short effective life for the disc.
 17. Theinvention of claim 13 or 15 wherein the reading-inhibit agent, onceactivated, provides a short practical usage lifetime for the disc.